1. Technical Field
This invention generally relates to a self monitoring satellite system and more specifically to a satellite having an on-board receiver and monitoring processor for observing and evaluating a signal sent from the satellite to determine the reliability of the signal. The invention also relates to providing a warning signal to indicate that particular satellite signals may be unreliable and to transmitting these warning signals at multiple frequencies to assist users in compensating for signal interference and ionospheric delays.
2. Background Art
The Global Positioning System (GPS) is a space-based radio positioning network designed to provide users who are equipped with a suitable receiver with position, velocity, and time (PVT) information. Developed by the United States Department of Defense (DOD), the space-based segment of GPS comprises a constellation of satellites in inclined 12-hour orbits around the earth.
FIG. 1 shows an exemplary constellation 100 of GPS satellites 101 in orbit around the earth. The GPS satellites 101 are nominally located in six orbital planes 102 with four GPS satellites 101 in each plane plus a number of xe2x80x9con orbitxe2x80x9d spare satellites (not shown) for redundancy. The orbital planes 102 of the GPS satellites 101 conventionally have an inclination of 55 degrees relative to the equator and an altitude of approximately 20,200 km (10,900 miles); each satellite completes one orbit in approximately 12 hours. This configuration positions the GPS satellites 101 so that a minimum of five of the GPS satellites 101 are normally observable (above the horizon) by a user anywhere on earth at any given time.
GPS provides PVT information based upon a concept referred to as time-of-arrival (TAO) ranging. The orbiting GPS satellites 101 each broadcast spread-spectrum microwave signals encoded with positioning data. The signals are conventionally broadcast at a number of known frequencies; for example, L1 at 1575.42 MHz, L2 at 1227.60 MHz, and (in the near future) L5 at 1176 MHz, with satellite ephemeris (satellite orbit data that allows its position to be computed in an earth-centered, earth-fixed, coordinate system) clock correction, and other data modulated using bi-phase shift keying, pseudo-random noise, or other techniques. Essentially, the signals are broadcast at precisely known times and at precisely known intervals. The signals are encoded with their precise times of transmission. A user receives the signals with a GPS receiver, which is designed to time the signals and to demodulate the satellite orbit data contained in the signals. Using the clock-correction data, the GPS receiver on earth determines the time between transmission by the satellite and reception by the receiver. Multiplying this amount by the speed of light gives what is termed the pseudorange measurement for that satellite.
If the GPS receiver clock were perfect, this would be the range measurement for that satellite from which the receiver could calculate its own PVT information. However, the imperfection of the clock causes the received measurement to differ from the satellite data by the time offset between actual time and receiver time. Thus, the measurement is called a pseudorange rather than a range. The time offset is common to the pseudorange measurements of all the satellites tracked by that receiver. By determining the pseudoranges of four or more satellites, the GPS receiver is able to determine its location in three dimensions, as well as the time offset. Thus, a user equipped with a proper GPS receiver is able to determine his PVT with great accuracy and to use this information to safely and accurately navigate from point to point, among other uses.
The accuracy of a user""s PVT is of varying importance to different classes of users. For example, a pilot of an airplane flying high over the ocean may not be particularly concerned to learn that the location calculated for the airplane from the GPS signal at a given time is off by 300 meters because that magnitude of error is not a safety concern to the airplane at that location. Contrarily, a fighter jet landing on an aircraft carrier or a passenger plane landing on a runway must have measurements accurate to within less than a meter, or the results may be fatal. Nevertheless, with conventional GPS systems, the same GPS signals are sent to all users. To address the need to have highly accurate, reliable GPS satellite signals, GPS signals are continuously measured by ground monitoring stations located on the earth. Each ground monitoring station includes a receiver which receives the satellite signal and evaluates it independently and in comparison with other satellite signals to determine the reliability of the satellite signal. Two examples of ground monitoring station types are Wide Area Augmentation Systems (WAAS) and Local Area Augmentation Systems (LAAS). The WAAS and LAAS stations each monitor multiple satellites and broadcast the signals from the satellites to users. If the Air Force, for example, is notified by the GPS ground monitoring stations that the satellite signal is unreliable, the GPS Operational Control Segment (OCS) transmits a signal back to the satellite to stop the satellite from transmitting unreliable signals until the satellite may be relied upon again. This takes a minimum of minutes and more typically hours to take this action. The OCS also may instruct the satellite to transmit a warning signal to users to indicate that the satellite""s signal is unreliable.
Signal reliability may be affected by any number of factors such as failure or space radiation upset within the satellite circuitry, inaccurate positioning data from the satellite, signal distortion, signal power level, atmospheric interference, timing errors, and the like. With the WAAS and LAAS ground stations monitoring the reliability of the satellite signals, however, loss of signal continuity and, thus, loss of a reliable signal may have one of two primary causes: 1) a detected or obvious failure (loss of signal) of one or more satellite signal measurements; and 2) a false alarm issued by an integrity monitor at the ground station, causing incorrect exclusion of one or more measurements. When a false alarm occurs, the ground station does not know where the error has occurred; namely, whether the error was caused within the satellite circuitry, signal interference between the satellite and the ground station, or within the ground station itself. Thus, the satellite signal may not be unreliable, but users are denied access to the satellite signal because the ground station has identified the signal as unreliable. For situations where the satellite signal is not safety critical, this extreme measure may not have been necessary. Loss of signal continuity in safety critical applications, such as civil aviation, for example, requires a civil aviator to abort its operation(s) after checking that the service which was predicted to be available at the start of the operation(s) is no longer available.
Additionally, for users of GPS signals in situations where the accuracy of the user""s PVT is a safety concern, the reliability of the satellite signal must be checked regularly. Using current technologies, it takes a minimum of 10 to 16 seconds to receive a confirmation of the reliability of a satellite signal. This is because when there is an error within the GPS system, the PVT information which is sent from the satellite must be checked at the ground monitoring station, and a signal must be sent back to a non-GPS satellite or other communication medium indicating that there is an anomaly. In safety-critical situations, where a delay of even 3 seconds may be fatal, the presently achievable minimum of 16 seconds is unacceptable.
As a result of this delay, other technologies have been developed in an attempt to provide more accurate, quicker responses for GPS signal reliability confirmation. One particular example is an Integrity Beacon Landing System (IBLS), such as that disclosed in U.S. Pat. No. 5,903,236 to Lawrence (May 11, 1999) which includes a pair of landing beacons located adjacent an airplane runway to create a reference xe2x80x9cbubblexe2x80x9d around a section of airspace to quickly orient an airplane to the correct PVT information required for that runway. Systems such as these, however, still maintain inherent delays and inaccuracies, and require additional hardware to be installed at each local site.
Thus, there is a need for a GPS satellite system which can quickly and accurately verify the reliability of each satellite signal and which can provide a GPS signal to users with very high continuity in addition to reliability.
The present invention relates to a self-monitoring satellite system configured to quickly and accurately convey satellite signal information and verification of its reliability to a user of the signal. Primarily, two aspects of the invention are described herein. First, to quickly verify the reliability of a satellite signal transmitted from a satellite, a receiver for monitoring the signal is included on the satellite. The receiver may receive signals transmitted from the satellite to which it is attached and perform any number of reliability tests on the signal to ensure its integrity. As used herein, the meanings of the terms xe2x80x9cintegrityxe2x80x9d and xe2x80x9creliabilityxe2x80x9d are intended to be interchangeable and are intended as an indication of whether a signal is trustworthy, meaning it is not corrupted or misleading. Second, to quickly warn users of an unreliable satellite signal, a warning signal is transmitted to users substantially simultaneous with the satellite signal. When reliability testing of a satellite signal is completed, an indication that the signal is unreliable may be sent on the same frequency as the initial satellite signal. Therefore, the time lag between the delivery of the satellite signal and the confirmation of its reliability is dependent only upon the update rate and response time of the signal tests.
An embodiment of the invention is configured as a GPS receiver that normally provides position, velocity, and time (PVT) information to a user. To check the reliability of the transmitted PVT information, it is received and monitored at a receiver on the satellite transmitting the signal. A plurality of tests are performed on the satellite signal and PVT information contained therein to ensure its reliability or integrity. If the signal is unreliable, a warning signal is sent as part of the next update of the satellite signal. Users of the PVT information, therefore, can reject a satellite""s unreliable signal or at a minimum consider the warning in deciding whether or not to rely upon the signal. The GPS satellite signals, and any warning signals sent simultaneous therewith, are simultaneously transmitted at each of three different frequencies. A receiver configured to receive the signals separately evaluates each of the three frequencies, calculates and compensates for any interference or ionospheric delay experienced by the satellite signal, and separates the various components of the signal. Using this and other satellite signals, a receiver may calculate the PVT information for the user and display any appropriate warnings regarding the reliability of the calculation made.
The foregoing and other features and advantages of the present invention will be apparent from the following more detailed description of the particular embodiments of the invention, as illustrated in the accompanying drawings.